A mixer is used in radio receiver architecture to translate a signal from a given center frequency to another. A direct conversion receiver (DCR) is a radio receiver design that translates the frequency of an incoming radio signal using a local oscillator (LO) whose frequency is identical to, or very close to, the carrier or center frequency of an intended signal. Accordingly in a DCR, the input radio frequency (RF) signal center frequency is converted to essentially zero Hertz (Hz). Recent developments in radio receiver art have shown that in the DCR, when compared to other passive or active mixer structures, a commutating mixer (also known as a Tayloe mixer) can achieve significant improvements in gain, noise figure, and intermodulation distortion performance.
FIG. 1 is a schematic diagram of a conventional single-balanced Tayloe mixer configuration as could be used for direct conversion to baseband in a DCR or a zero intermediate frequency (IF) receiver. This configuration provides excellent performance for the DCR architecture, in which the LO frequency is set essentially equal to the center frequency or carrier frequency of the received RF signal. The Tayloe mixer provides a low-pass response IF in-phase (I) and quadrature phase (Q) outputs. An essential feature of the Tayloe mixer is that the duty cycle of each of the four LO waveforms is near twenty five percent (25%), as shown in FIG. 1. In fact, the duty cycle can be considerably less than twenty five percent (25%) with relatively little impact on performance However, the gain, noise figure, and other performance criteria begin to degrade as the duty cycle increases above twenty five percent (25%).
Its inherent low-pass IF response makes the Tayloe mixer unsuitable for use in radio receiver applications that require a heterodyne mixer. In a heterodyne mixer, the input RF signal center frequency and the LO frequency are separated by a relatively large difference, which is equal to the IF of the mixer output signal. For a heterodyne mixer where the IF output may be above 100 mega-Hertz (MHz), the low-pass response of the Tayloe mixer makes it disadvantageous when compared to a standard switching mixer using fifty percent (50%) duty cycle LO waveforms and with no capacitance added at the mixer output terminals.
In the Tayloe mixer shown in FIG. 1, the RF input signal is transferred sequentially during four quarter LO period timeslots by four switches 102a-102d to four output capacitors 104a-104d and the I and Q outputs are taken differentially. This is the simplest balanced commutating IQ mixer structure, and is “single balanced” in the sense that I and Q output signals are differential, while the RF input is single ended. A “double balanced” structure, as shown in FIG. 2 may also be used. The double balanced structure, shown in FIG. 2, provides for differential RF input, and is formed by providing four (4) additional switches 102e-102h from the negative side of the RF source.
For a variety of reasons, some radio receiver applications may require a heterodyne architecture, as opposed to direct conversion architecture. FIG. 3 is a schematic diagram of a conventional double balanced switching IQ mixer, which uses fifty percent (50%) duty cycle complementary LO switching waveforms. This configuration can be used in DCR as well as heterodyne mixing applications. While I and Q mixers are shown, as would be used in a DCR, a single mixer would normally be used for heterodyne mixer applications, where only one output is usually needed. Only a small amount of IF output capacitance is tolerable with the mixer shown in FIG. 3. Note the similarity between the double balanced IQ Tayloe mixer, shown in FIG. 2, and the double balanced switching mixer shown in FIG. 3. The primary differences between the mixers of FIG. 2 and FIG. 3 are the use of twenty five percent (25%) duty cycle LO waveforms plus the addition of the output capacitors 104 on the Tayloe mixer. Nevertheless, these seemingly minor differences have proven to effect a significant improvement in several key mixer performance criteria, such as noise figure, gain, and third order inter-modulation intercept point in DCR implementations.
Heterodyne mixers normally respond equally to RF input signals either above or below the LO frequency separated by a frequency difference equal to the IF. However, only one of these responses is normally desired. In addition to creating an undesired response (image response) at the image frequency, the noise at the image frequency is also converted to the IF output, which degrades the noise figure of a heterodyne mixer by up to three (3) dB.
Accordingly, there exists a need for new heterodyne mixer structures that achieve significant improvements over current heterodyne commutating mixers and provide for an efficient and low cost implementation of image rejection.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. One experienced in the art will appreciate that details such as possible use of a balun to drive the differential RF input port, and some DC biasing elements have been left off the drawings for the sake of clarity. In each of the drawings, the RF signal source is shown as a voltage source Vs along with a representation of its inherent source impedance, Rs.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.